Shear relief check valve for flushing passageways of earth penetrating tools



July 19, 1966 w. A. KISTLER, JR 3,

SHEAR RELIEF CHECK VALVE FOR FLUSHING PASSAGEWAYS 0F EARTH PENETRATING TOOLS Original Filed Nov. 1, 1962 FIGURE 4 WILLIAM A. KISTLER, J R.

INVENTOR.

BYWM)2 ATTORNEY United States Patent 3,261,413 SHEAR RELIEF CHECK VALVE FOR FLUSHING PASSAGEWAYS OF EARTH PENETRATING TOOLS William A. Kistler, Jr., Houston, Tex., assignor to Hughes Tool Company, Houston, Tex., a corporation of Delaware Original application Nov. 1, 1962, Ser. No. 234,664, now Patent No. 3,213,949, dated Oct. 26, 1965. Divided and this application Feb. 11, 1965, Ser. No. 431,799 6 Claims. (Cl. 175317) The present application has been divided from that co-pending application bearing Serial Number 234,664, filed November 1, 1962, now Patent No. 3,213,949, issued on October 26, 1965. The present invention lies in the field of auxiliary devices used with earth penetrating tools, in particular devices used to obtain a measure of control over the flow of flushing fluid used in drilling with such tools. Such a tool is typically suspended from the lower end of a long string of hollow drill pipe through which a flushing fluid or mud is circulated down to the bit and jetted therefrom through a number of nozzles or watercourses. This fluid is then circulated upwardly through the annulus between drill pipe and borewall, carrying the rock cuttings to the surface.

In recent years it has become a common practice to use large pressure drops over such tools to obtain high velocity streams of flushing fluid striking the bottom of the borehole. This is done primarily by using replaceable nozzles with quite small orifices therein, e.g., as small as A" or in a Tricone 3-cone rotary bit of 8 /8" diameter having three flushing fluid passageways, such nozzles being replaced by others with a different orifice diameter under a diflerent set of drilling conditions (type and depth of formation, rotary speed, weight on bit, mud weight, etc.). While such small orifice nozzles have served admirably their intended function of providing high velocity mud stream on bottom, they also create special problems. The small openings are likely to become easily plugged, and they are not well adapted to the circulation of lost circulation materials, i.e., coarse materials circulated down the drill string and bit to plug a porous zone which threatens to absorb all of the drilling fluid. They also create special problems when the bit is being run into the hole or pulled from the hole, as the mud standing in the string does not flow into or out of the nozzle openings at a high rate of speed. It is usually necessary to lower or raise the drill string and bit rather slowly to avoid pressure surges which may cave in the wall of the formation.

The present invention provides a means for overcoming some of the named disadvantages associated with jet bits, primarily by providing in place of one nozzle (or more) a special member which is both automatically expellable if the nozzles should become plugged and is controllably expellable in other circumstances. Such special member includes a core or nozzle releasably and sealably secured in the lower end of a bit flushing fluid passageway and having a downwardly extending opening therethrough, a a movable member such as a ball seating in the entrance end of such opening to prevent the downward flow of flushing fluid, means to restrain the upward movement of such movable member from a position just above its seating position and yet allowing such limited movement under the influence of a reverse circulation of flushing fluid, and one or more shear pins securing the core to the passageway wall or securing parts of a multipiece core together in such manner that at least a portion of the core may be expelled upon a shearing of the pins. With normal, downward circulation the assembly is activated upon an increase in the normal pressure drop over the bit to a boss of a rock bit,

ice

critical or design value, under which condition the pin or pins are sheared and the expellable portion of the core is washed out. Such activation occurs automatically upon a plugging of the operating nozzles of the bit, or controllably by speeding up the mud pump which circulates the flushing fluid.

It can thus be seen that the present invention achieves several objects:

(1) Prior to activation, assuming the bit to be at the bottom of the hole, it blocks normal downward flow through the flushing passageway in which it is installed, thus permitting the use of a lesser number of nozzles of greater orifice than would be the case if nozzles were used in all passageways and thereby reducing the likelihood of unplanned plugging, while at the same time permitting reverse, upward flow under a negative gradient. If such negative gradient occurs when the operating n-ozzles are plugged and the bit is at the bottom of an extremely deep well, there is a hazard of drill stem collapse because of the high pressure of fluids in the annulus around the drill stem. Such hazard is eliminated when the present invention is employed.

(2) When running a bit into a hole, the shear relief check valve of the present invention permits upward flow of the drilling fluid through its own opening. To better accomplish this object, such opening may be of considerably larger section than the nozzle orifices, with the result that the drilling mud which is standing in the hole flows relatively rapidly into the drill stem, consider-ably reducing the pressure surges otherwise felt.

(3) When activated by a shearing of the shear pin and expulsion of all or part of the core, the valve operates to relieve plugged nozzles and permit normal circulation of drilling mud, either alone or with lost circulation materials.

(4) In preparing to pull a bit when there has been no prior occasion to activate the valve, it may be activated to reduce the pressure surges otherwise manifested if the bit is pulled too rapidly. The extra opening thus provided permits a more rapid draining of the mud from the drill stem and makes rapid pulling possible.

The present invention may be more readily understood by reference to the accompanying drawing, in which:

FIGURE 1 illustrates in vertical cross section a preferred embodiment of the present invention as installed in a portion of an earth penetrating tool, e.g., a nozzle FIGURE 2 is a perspective view of the ball retainer of FIGURE 1,

FIGURE 3 is a vertical cross-section of a modification of the embodiment of FIGURE 1, differing therefrom in that the shear pin underlies the entire core of the valve and is supported by the nozzle boss rather than by a sleeve releasably secured between the boss and the core,

FIGURE 4 depicts in vertical section an alternate embodiment in which a different means is used to retain the ball in place, this view showing the ball in the raised position it would occupy during reverse circulation, and

FIGURE 5 is another vertical section of the FIGURE 4 embodiment, on a sectioning plane disposed at right angles to that 'of FIGURE 4, the only other difference being that the ball is Shown in seating position, as during normal downward circulation.

Referring now to IFIGURES 1 and 2, the embodiment there shown is disposed in nozzle boss 1 having flush-ing passageway 2 therethrough, such passageway vbeing oounterbored from its lower end 3 to form shoulder 4. Ball retainer or wire cage '5, comprising the two U-shaped inverted wires or rods 6 secured together by welding at their closed ends 7 and with their free ends bent to form outwardly extending hooks 8, is inserted in flushing p assageway 2 so that hooks 8 are engaged by shoulder 4.

Two U-shaped wires 6 are used to prevent the upward movement of the ball 9 in the spaces the-rebetween, but more may be used as necessary.

Core assembly 11, having an outside diameter only slightly less than the diameter of the counterbore, is slidably mounted in the lower end of passageway 2 to press hooks 8 of ball retainer against shoulder 4, core assembly 11 being secured against downward movement by a conventional snap 'ring 12 lying partly in the indicated groove in boss 1 and partly underlying the bottom of core assembly 1 1. A conventional O-ring 13 is used to prevent leakage between core assembly 11 and the counterbore of boss 1, such O-ring 13 lying either in the indicated groove in the borewall, in a similar groove in the outer periphery of the core assembly, or in registering half-grooves in both members.

The core assembly 11 of FIGURE 1 consists of an expellable inner member or core 14 and an outer member or shell 15, more 14 being slidable within shell 15 and in sealing fashion by virtue of O-ring '16. Core 14 has an axial opening 17 therethrough, flared at the upper end to provide a seat 18 for ball 9. The two members are secured together by shear pin 19 extending through the indicated registering openings extending transversely through their walls. It should be noted that in the complete assembly snap ring 12 underlies only the shell 15 of core assembly 11.

The modification of FIGURE 3 diflers from that of FIGURE 1 in that the shear pin 19 underlies core assem bly 11' and extends into holes in the nozzle boss 1. While this assembly is shown as consisting of the separate core 14' and sleeve or shell 15', it is apparent that these two pieces may be integrated into one and that O-rin-g 16 may be eliminated. No snap ring is necessary, of course, as shear pin 19 serves the dual function of securing the core in the nozzle boss and acting as the rupturable member.

It may also be noted that the core assembly 11 of FIG- URE 1 may be made with core 14 and shell 15 integrated into a single piece, in which case the shear pin 19 may be extended through appropriate openings in nozzle boss 1. It will be apparent that in either modification it is unnecessary to have the shear pin extend across passageway 17 and that separate pins may be used extending only across the interfaces where shearing is to take place, or that only one such pin may be provided, so long as the pin or pins extend sufliciently far into the confronting members that adequate purchase is furnished. It will also be appreciated that it is not essential that passageway 2 be provided in two parts of diflerent diameters, i.e., it may be of the diameter of the counterbore throughout. In such event, retainer 5 is welded to the top of core assembly 11 or 11', and in FIGURE 1 the core assembly 11 is secured to the nozzle boss 1 by the type snap ring assembly disclosed in the application of Kis-tler, S.N. 702,060, filed December 11,1957, now abandoned, or in Mandrell, U.S. Patent No. 3,115,200, and described in connection with the FIGURE 4 embodiment below.

In the embodiment of FIGURES 4 and 5, the shear pin 19 serves as the rupturable member permitting expulsion of core 14 from sleeve 15 and cooperates with the link or eye screw 21 to restrict the upward movement of ball 9'. The shaft or stem 23 of the eye screw is preferably threaded and engages like threads in ball 9' to secure the two members together, but may be extended therethrough and secured by other conventional means, particularly if the ball is composed of such highly resilient material that a threaded engagement is not feasible.

FIGURE 4 show-s ball 9' in the extreme upward position it would occupy under the influence of violent reverse circulation and the restraining effect of the contact between eye screw 21 and shear pin 19, while FIGURE 5 shows the ball in its normal seating position in the downwardly converging wall or seat 18 defining the upper end of opening 17. These figures also illustrate the above mentioned Kistler structure for retaining core assembly 11 in the flushing fluid passageway. Such structure requires the indicated registering grooves in the wall defining passageway 2 and in the outer periphery of core assembly 11" to accommodate snap ring 12. An axial access slot 22 is provided in the outer portion of the core assembly (or in boss 1 or partly in both) for the insertion of a tool to engage the tree ends of the ring. When the core is to be inserted or removed from the boss, snap ring 12 is compressed or expanded by such tool so that the snap ring lies Wholly within one or the other of the pair of pair of registering grooves. With such retaining structure, passageway 2 may be of uniform bore throughout, eliminating shoulder 4, as the snap ring retains the assembly against both upward and downward movements.

In other respects the FIGURES 45 embodiment is similar to those of FIGURES 1-3, and the many variations described in connection with the latter group may be applied to the former. As one instance, the snap ring shown in FIGURES 4-5 may be eliminated and replaced with a shear pin underlying the core assembly 11" and extending into nozzle boss 1, as in FIGURE 3 (and obviously the replacement may also be the type snap ring as sembly shown in FIGURES 1 and 2). With such doublefunction underlying shear pin, it is only necessary to extend the loop portion of eye screw 2-1 so that ball 9' will seat on entrance wall 18 under downward flow and float above it somewhat under reverse flow. With this modification and also with those analogous to the modifications discussed in connection with FIGURE 1 (shear pin or pins extending at least partially into openings in the nozzle boss 1 and partially into the registering openings in the core assembly), the core assembly 11 may be made in a single piece, integrating core 14 and sleeve 15 and eliminating O-ring :16. With respect to the embodiments shown in both g ro/ups of figures, the structure shown in 'FIGURE 1 and in FIGURES 4-5 are preferred because they are more readily inserted and withdrawn, and because the parts are less likely to become separated than with a separate shear pin which must be inserted through openings in the nozzle boss.

Balls 9 and 9' may be made of a wide variety of mate rials, including metals, relatively hard rubber, and the comparatively hard but somewhat resilient plastics such as nylon. The rubber-cover nylon balls sold under the proprietary name Frac balls are eminently suitable because they combine the indicated characteristics with relatively light weight, .a quantity of some importance when running a bit into a hole filled with mud.

Although balls 9 and 9' are illustrated only as members having spherical surfaces, it is apparent that such members may also have other outer surfaces, e.g., a downwardly converging conical surface, in which event seat 18 is provided with a similar surface. No problem is presented with respect to a movable sealing member having such shape in the FIGURES 4-5 embodiment, as the eye screw and shear pin interfit prevent the movable member from assuming a malfunctioning attitude. In the FIGURES 1-3 embodiments it is necessary to shape the movable member and the ball retainer assembly so that the former has no opportunity to become inverted or cocked.

Broadly the present invention is a shear relief check valve for a fluid flushing passageway of an earth penetrating tool which blocks the flow of flushing fluid downwardly through such passageway under pressure gradients below a design or critical value, permits the flow of flushing fluid upwardly under a negative pressure gradient, and is actuable under pressure drops exceeding such design value to permit the downward flow of flushing fluid.

While the illustrated assemblies have been described as including a core assembly disposed in a flushing fluid passageway of an earth penetrating tool, and such core assembly has been said to include an inner core and an outer sleeve, it is apparent that the sleeve member, when used, may with equal facility be considered a part of the earth penetrating tool, that part which defines the lowermost portion of one of its flushing fluid passageways. With this in mind and also keeping in mind that a sleeve is not essential when the shear pin extends into the nozzle boss and the core is made large enough fiully to occupy the passageway in the boss, the part of the tool in which the core is sea-ted may be described as a passageway subassembly consisting of'at least one member in the form of a nozzle boss or other integral fluid delivery portion of the tool, as such language implicitly states that such subassembly may consist only of the nozzle boss or may consist of the nozzle boss in combination with a sleeve.

As used in the appended claims, the term positive pressure gradient means, when used in reference to the fluid flow cap-ability of a passageway in an earth penetrating tool, a pressure differential between the inside of the passageway and the space outside of the passageway adjacent its exit end such that the pressure inside the passageway is higher than the outside pressure. Similarly, negative pressure gradient means that the pressure outside of the passageway and adjacent its exit end is higher than the pressure inside the passageway.

In the claims:

1. In an earth penetrating or similar tool having a number of flushingfl-uid passageways therethrough with an entry end for receiving flushing fluid delivered through communicating passages in the tool and an exit end for discharging such fluid from the tool, each such passageway having an axial or flow dimension and a dimension transverse such flow dimension, each such passageway being thus adapted to receive flushing fluid and deliver a forward flow thereof externally of the tool under the influence of a positive pressure gradient, and also to supply reverse flow into such tool under a negative pressure gradient, the part of such passageway above and adjacent its exit end being defined by the inner wall of a passageway subassembly consisting of at least one member in the form of a nozzle boss or other integral fluid delivery portion of said tool having one of said passageway therethrough, a shear relief check valve comprising,

(A) a core or nozzle in said passageway and slidable from the exit end thereof and disposed in sealing relationship with said inner wall of the passageway subassembly at the outer periphery of the core, said core having a central opening therethrough with entry and exit ends respectively adjacent the corresponding ends of the passageway, said entry end being smaller than the transverse section of the tool passageway and being shaped to provide a seat for a movable obturating member or ball,

(B) a movable obturating member or ball disposed above the entry end of said core opening and adapted to seat therein in sealing relationship therewith, said movable member cooperating with said core to block the forward flow of flushing fluid therethrough under positive pressure gradients, such being the blocking position of the movable member, and being movable from said core opening toward the entry end of said tool passageway to a non-blocking position under the influence of negative pressure gradients to permit the reverse flow of flushing fluids,

(C) a shear pin extending transversely into said inner wall of the passageway subassembly and across said core opening in supporting contact with aid core and underlying at least a portion of the core lying between its entry end and its exit end, said shear pin thus supporting said core against expulsion from the exit end of the passageway at all positive pressure gradients below a predetermined critical value and being shearable at pressures above said critical value to permit the expulsion of said core and the flow of flushing fluid through said passageway, and

(D) retainer means in the form of a wire link attached to said obturating member and extending through said core opening and around said shear pin, the lower part of said link contacting the shear pin when the obturating means is spaced above said seat to define said non-blocking position and being spaced from the shear pin in the blocking position of the obturating member.

2. In the earth penetrating or similar tool of claim 1 in which said passageway subassernbly consists of the combination of said nozzle boss or other similar fluid delivery portion having a central channel therethrough communicating with the tool passageway together with a sleeve releasably secured within said' boss in sealing contact between the outer periphery of the sleeve and the channel wall of the boss, said sleeve having an inner periphery constituting said wall of the passageway subassernbly, the shear relief valve of claim 1 in which said shear pin extends at least into said sleeve.

3. In the earth penetrating or similar tool of claim 2, the shear relief valve thereof in which said shear pin terminates within said outer periphery of the sleeve.

4. In an earth penetrating or other tool having a fluid flow passageway therethrough terminating in an exit end, a shear relief check valve comprising a sleeve disposed in said passageway adjacent its exit end, said sleeve as thus disposed being releasably secured to said tool in sealing relationship with the wall of said passageway, a nozzle member disposed within said sleeve in sealing relationship therewith and slidable from said sleve at the exit end of the passageway, said nozzle having an axial opening therethrough with an exit end disposed adjacent the exit end of the passageway and an entry end within the passageway flared to define a seat for a movable obturating member disposed thereabove, a movable obturating member of lesser cross section than said passageway and greater than said nozzle opening disposed above and adjacent said entry end of the nozzle opening and adapted to seat therein to block the flow of fluid from said passageway into the entry end of said opening, a shear pin extending at least partially across said nozzle opening and into registering openings in said nozzle and sleeve, said shear pin thus securing the nozzle to the sleeve at fluid pressures below a predetermined critical value and being sheara-ble at higher values to permit the expulsion of said nozzle from the exit end of the passageway, and a ball retainer in the form of a wire having one end secured to said obturating member and the other bent in a loop passing between said shear pin and the exit end of the nozzle opening in the blocking position of the obturating member, said other end of the wire contacting the shear pin when the obturating member is spaced above said seat to prevent its being moved further under the influence of a negative pressure gradient.

5. A shear relief check valve adapted for slidable insertion in the portion of a fluid passageway of an earth penetrating or similar tool adjacent the exit end thereof, where fluid is normally delivered by said tool, so that the outer periphery of said valve is disposed in sealing relationship with the passageway-defining wall of said tool, said valve comprising an outer sleeve member and an inner core member the sleeve member having said outer periphery of the valve and being adapted to be releasably secured to said tool, the core member being sealingly disposed within said sleeve and slidable from said sleeve toward said exit end of the passageway, said core having a central opening therethrough adapted to be disposed with its exit end adjacent the exit end of the passageway and its entry end within the passageway, said entry end diverging outwardly from the center of the opening to define a seat for a movable obturating member, a movable obturating member disposed above said seat of lesser cross sec-tion than said passageway and larger than said opening and adapted to seat in said entry end to prevent fluid flow from said passageway into said opening, a shear pin extending between said sleeve and core through registering openings in each, and across said opening, and a ball retainer in the form of "an eye screw having a stem portion secured to said obturating member and an eye port-ion connected to the stern and surrounding and spaced from said shear pin in the blocking position of the obturating member, said eye screw being movable toward said passageway with the obturating member under the influence of a negative pressure gradient and engaging the shear pin in said non-blocking position to prevent turther movement of the 'obturating member.

6. A shear relief valve sub assembly adapted to be aXia-lly inserted in and sealingly secured to the Wall of a flushing fluid passageway of an earth penetrating tool containing a multiplicity of such passageways to permit reverse flow through such passageway, to prevent forward flow therethrough under nonmal positive pressure gradicuts, from inside said passageway to the outside of said tool, and to allow such forward flow at higher such positive pressure gradients, comprising in combination:

an assembly consisting essentially of an outer shell and an inner core, the outer periphery of said shell being adapted to sealingly engage the wall of said passageway and to be releasably secured with respect thereto, said core being slidably inserted within said shell and sealingly disposed therein, said core having an opening extending axially therethrough with a seat at the upper end,

a shear pin extending transversely across the axial opening in the core and through registering openings in said shell and said core,

5 said movable member and an axially elongated eye portion extending downwardly therefrom into the opening in said core, said stem portion being shorter than the axial distance between the shear pin and the movable member as seated in said opening, said eye 10 of the retainer surrounding said shear pin to permit the movable member to move a limited distance upward.

References Cited by the Examiner UNITED STATES PATENTS 2,945,678 7/1960 Boud-reaux 1753 17 2,987,130 6/1961 McIntyre 175340 X 3,012,623 12/1961 Powers 175-318 20 3,087,558 4/1963 Dou'gherty 175-317 X 3,096,834 7/1963 Steen 175-422 3,189,107 6/ 1965 Galle 175-422 X 25 CHARLES E. OCONNELL, Primary Examiner.

R. E. FAVREAU, Assistant Examiner. 

1. IN AN EARTH PENETRATING OR SIMILAR TOOL HAVING A NUMBER OF FLUSHING FLUID PASSAGEWAYS THERETHROUGH WITH AN ENTRY END FOR RECEIVING FLUSHING FLUID DELIVERED THROUGH COMMUNICATING PASSAGES IN THE TOOL AND AN EXIT END FOR DISCHARGING SUCH FLUID FROM THE TOOL, EACH SUCH PASSAGEWAY HAVING AN AXIAL OR FLOW DIMENSION AND A DIMENSION TRANSVERSE SUCH FLOW DIMENSION, EACH SUCH PASSAGEWAY BEING THUS ADAPTED TO RECEIVE FLUSHING FLUID AND DELIVER A FORWARD FLOW THEREOF EXTERNALLY OF THE TOOL UNDER THE INFLUENCE OF A POSITIVE PRESSURE GRADIENT, AND ALSO TO SUPPLY REVERSE FLOW INTO SUCH TOOL UNDER A NEGATIVE PRESSURE GRADIENT, THE PART OF SUCH PASSAGEWAY ABOVE AND ADJACENT ITS EXIT END BEING DEFINED BY THE INNER WALL OF A PASSAGEWAY SUBASSEMBLY CONSISTING OF AT LEAST ONE MEMBER IN THE FORM OF A NOZZLE BOSS OR OTHER INTEGRAL FLUID DELIVERY PORTION OF SAID TOOL HAVING ONE OF SAID PASSAGEWAYS THERETHROUGH, A SHEAR RELIEF CHECK VALVE COMPRISING, (A) A CORE OR NOZZLE IN SAID PASSAGEWAY AND SLIDABLE FROM THE EXIT END THEREOF AND DISPOSED IN SEALING RELATIONSHIP WITH SAID INNER WALL OF THE PASSAGEWAY SUBASSEMBLY AT THE OUTER PERIPHERY OF THE CORE, SAID CORE HAVING A CENTRAL OPENING THERETHROUGH WITH ENTRY AND EXIT ENDS RESPECTIVELY ADJACENT THE CORRESPONDING ENDS OF THE PASSAGEWAY, SAID ENTRY END BEING SMALLER THAN THE TRANSVERSE SECTION OF THE TOOL PASSAGEWAY AND BEING SHAPED TO PROVIDE A SEAT FOR A MOVABLE OBTURATING MEMBER OR BALL, (B) A MOVABLE OBTURATING MEMBER OR BALL DISPOSED ABOVE THE ENTRY END OF SAID CORE OPENING AND ADAPTED TO SEAT THEREIN IN SEALING RELATIONSHIP THEREWITH, SAID MOVABLE MEMBER COOPERATING WITH SAID CORE TO BLOCK THE FORWARD FLOW OF FLUSHING FLUID THERETHROUGH UNDER POSITIVE PRESSURE GRADIENTS, SUCH BEING THE BLOCKING POSITION OF THE MOVABLE MEMBER, AND BEING MOVABLE FROM SAID CORE OPENING TOWARD THE ENTRY END OF SAID TOOL PASSAGEWAY TO A NON-BLOCKING POSITION UNDER THE INFLUENCE OF NEGATIVE PRESSURE GRADIENTS TO PERMIT THE REVERSE FLOW OF FLUSHING FLUIDS, (C) A SHEAR PIN EXTENDING TRANSVERSELY INTO SAID INNER WALL OF THE PASSAGEWAY SUBASSEMBLY AND ACROSS SAID CORE OPENING IN SUPPORTING CONTACT WITH SAID CORE AND UNDERLYING AT LEAST A PORTION OF THE CORE LYING BETWEEN ITS ENTRY END AND ITS EXIT END, SAID SHEAR PIN THUS SUPPORTING SAID CORE AGAINST EXPULSION FROM THE EXIT END OF THE PASSAGEWAY AT ALL POSITIVE PRESSURE GRADIENTS BELOW A PREDETERMINED CRITICAL VALUE AND BEING SHEARABLE AT PRESSURES ABOVE SAID CRITICAL VALUE TO PERMIT THE EXPULSION OF SAID CORE AND THE FLOW OF FLUSHING FLUID THROUGH SAID PASSAGEWAY, AND (D) RETAINER MEANS IN THE FORM OF A WIRE LINK ATTACHED TO SAID OBTURATING MEMBER AND EXTENDING THROUGH SAID CORE OPENING AND AROUND SAID SHEAR PIN, THE LOWER PART OF SAID LINK CONTACTING THE SHEAR PIN WHEN THE OBTURATING MEANS IS SPACED ABOVE SAID SEAT TO DEFINE AND NON-BLOCKING POSITION AND BEING SPACED FROM THE SHEAR PIN IN THE BLOCKING POSITION OF THE OBTURATING MEMBER. 